Synthesis, structure and reactivity of N,O-metallacyclic (dicarbonyldiazene) platinum complexes

Abstract

Reaction of cis-Pt(PR¹₃)₂Cl₂ (R¹=Me, Ph) with diacyl hydrazine R²OCNHNHCOR² (R²=Me, Ph) in refluxing ethanol and NaHCO₃ afforded platinum diazene diacyl complexes Pt(PR¹₃)₂(R²OCNNCOR²) (1, R¹=R²=Ph; 3, R¹=Me, R²=Ph; 4, R¹=R²=Me). Reaction of Pt(PPh₃)₂(C₂H₄) with diazene dicarboxylates R²OCNNCOR² (R²=OEt, OPrⁱ) resulted in the formation of Pt(PPh₃)₂(R²OCNNCOR²) (2, R²=OEt; 5, R²=OPrⁱ). Multinuclear NMR spectroscopy on 1–5 revealed that the dicarbonyl-substituted diazene ligand is coordinated asymmetrically, consistent with a five membered Pt–N–N–C–O ring. Compound 5 crystallises from THF–benzene as the solvate Pt(PPh₃)₂(PrⁱO₂CNNCO₂Prⁱ)·(C₆H₆)₂; its X-ray crystal structure shows that the coordination sphere of platinum is essentially square planar and coplanar with the five membered Pt1–O1–C5–N2–N1 ring. The Pt1–O1 distance is 2.027(4) Åand the Pt1–N1 distance is 2.050(5) Å. The N1–N2 distance is 1.421(6) Åwhile the N2–C5 and C5–O1 distances are 1.273(7) and 1.314(7) Å, respectively. Complexes 1–5 show sensitivity towards chlorinated solvents (CH₂Cl₂, CHCl₃) under photolysis conditions, forming the corresponding cis-platinum bis(phosphine) dichloride complexes; the same products are formed in a slower thermal reaction with 2 and 5. Complexes 2 and 5 react photochemically with ethylene in [²H₈]THF yielding Pt(PPh₃)₂(C₂H₄) but 1, 3 and 4 are inert. Addition of an excess of dppe ligand to 2 and 5 resulted in the displacement of PPh₃ and the formation of Pt(dppe)(EtO₂CNNCO₂Et) and Pt(dppe)(PrⁱO₂CNNCO₂Prⁱ) [dppe=1,2-bis(diphenylphosphino)ethane].